Thermally conductive sheet and process for producing same

ABSTRACT

A thermally conductive sheet has cut surfaces with low surface roughness and hence shows reduced thermal resistance at the interfaces, and high thermal conductivity in the thickness direction. Thus, the thermally conductive sheet can be interposed between any of various heat sources and a radiation member. The process for producing the thermally conductive sheet includes at least: an extrusion molding step in which a thermally conductive composition containing a polymer, an anisotropic thermally conductive filler, and a filler is extruded with an extruder to thereby mold an extrusion-molded product in which the anisotropic thermally conductive filler has been oriented along the extrusion direction; a curing step in which the extrusion-molded product is cured to obtain a cured object; and a slicing step in which the cured object is sliced into a given thickness with an ultrasonic cutter in the direction perpendicular to the extrusion direction.

FIELD OF THE INVENTION

This invention relates to a thermally conductive sheet and a process forproducing such a thermally conductive sheet. The present applicationasserts priority rights based on JP Patent Application 2010-138334 andJP Patent Application 2010-138417 filed in Japan on Jun. 17, 2010, aswell as JP Patent Application 2011-79976 filed in Japan on Mar. 31,2011. The total contents of disclosure of the patent applications of thesenior filing date are to be incorporated by reference into the presentapplication.

BACKGROUND OF THE INVENTION

Along with higher performances achieved in electronic apparatuses,semiconductor elements with a higher density and highly packagedsemiconductor elements have been developed. In response to this trend,it becomes essential to more efficiently radiate heat generated fromelectronic parts forming electronic apparatuses. In order to allow thesemiconductor to radiate heat efficiently, the semiconductor is attachedto a heat sink, such as a heat radiating fin, a heat radiating plate, orthe like, with a thermally conductive sheet interpolated therebetween.As the thermally conductive sheet, such a sheet that is made of siliconewith a filler (thermally conductive filler), such as an inorganicfiller, contained and dispersed therein has been widely used.

In this heat radiating member, further improvement of a thermalconductivity is demanded, and in general, a filling rate of inorganicfiller blended in a matrix is increased so as to achieve a high thermalconductivity. However, when the filling rate of the inorganic filler isincreased, the flexibility is lowered, or spilled powder might occurbecause of the high filling rate of the inorganic filler, this methodfor increasing the filling rate of an inorganic filler has a limitation.

As the above-mentioned inorganic filler, for example, alumina, aluminumnitride, aluminum hydroxide, etc. are proposed. Moreover, in some cases,in order to obtain a higher heat conductivity, boron nitride (BN),scale-shaped particles, such as graphite, carbon fibers, etc. are filledinto a matrix. In this case, the anisotropic property of thermalconductivity possessed by the scale-shaped particles, etc. is utilized.For example, carbon fibers have a thermal conductivity of about 600W/m·K to 1200 W/m·K in the fiber direction. It has been known that theanisotropic property of boron nitride is such that a thermalconductivity of about 110 W/m·K is exerted in the plane direction and athermal conductivity of about 2 W/m·K is exerted in a directionperpendicular to the plane direction.

In this manner, the plane direction of carbon fibers or scale-shapedparticles is made to be the same as the thickness direction of the sheetthat is a heat transmitting direction. That is, by orienting the carbonfibers or the scale-shaped particles in the thickness direction of thesheet, it becomes possible to remarkably improve the thermalconductivity. However, in the case when, after having been molded, acured object having been subjected to a curing process is sliced into adesired thickness, since the cured object having flexibility is slicedwhile being deformed, concave/convex portions on the sheet surfacebecome greater, with air being involved into the concave/convexportions, resulting in a problem of failing to effectively utilize itssuperior thermal conductivity.

In order to solve the above-mentioned problem, for example, PatentLiterature 1 has proposed a thermally conductive rubber sheet that isformed by being punched out with blades that are aligned in a directionperpendicular to the sheet longitudinal direction with equal intervals,and then sliced. Moreover, Patent Literature 2 has proposed a method inwhich a laminated member, formed by stacking layers while repeatedlycarrying out coating and curing processes, is sliced with a cuttingdevice with a round rotary blade so that a thermally conductive sheetwith a predetermined thickness is obtained. Furthermore, PatentLiterature 3 has proposed a method in which a laminated member, formedby stacking two or more graphite layers containing anisotropic graphiteparticles, is cut with a metal saw so as to be oriented with 0° relativeto the thickness direction of the sheet so as to obtain an expandedgraphite sheet (with an angle of 90° relative to the stacked layersurface).

In these proposed cutting methods, however, since the surface roughnesson a cut surface becomes higher, a greater thermal resistance is causedon the interface, resulting in a problem of a reduction in thermalconductivity in the thickness direction.

Therefore, under these circumstances, there have been strong demands forproviding a thermally conductive sheet that has a reduced thermalresistance because of its small surface roughness on a cut surface witha high thermal conductivity in the thickness direction, and is suitablefor use as a member to be interposed between any of various heat sources(for example, various devices, such as a CPU, a transistor, and an LED)and a heat radiating member, and a method of producing such a thermallyconductive sheet.

PRIOR-ART DOCUMENTS Patent Document

-   PTL 1: Japanese Patent Application Laid-Open No. 2010-56299-   PTL 2: Japanese Patent Application Laid-Open No. 2010-50240-   PTL 3: Japanese Patent Application Laid-Open No. 2009-55021

SUMMARY OF THE INVENTION

The present invention has been devised to solve the above-mentionedconventional problems and aims to achieve the following object. That is,the object of the present invention is to provide a thermally conductivesheet in which because of its small surface roughness on a cut surface,the thermal resistance on the interface is reduced, and because of itshigh thermal conductivity in the thickness direction, it is suitable foruse as a member to be interposed between any of various heat sources anda heat radiating member, and a method of producing such a thermallyconductive sheet.

In order to solve the above-mentioned problems, the present inventorshave intensively carried out various researches and have reached thefollowing findings. That is, by allowing a thermally conductivecomposition containing an anisotropic thermally conductive filler and afiller to pass through a plurality of slits, the anisotropic thermallyconductive filler blended in the thermally conductive composition isoriented in the thickness direction of the thermally conductive sheet,and molded without disturbing the oriented state of the anisotropicthermally conductive filler, and then extrusion-molded through a moldoutlet as a block body.

After the resulting molded product has been cured, the cured object iscut in a direction perpendicular to the extrusion direction by using anultrasonic cutter into a given thickness; thus, it has been found thatsince cut surfaces with low surface roughness are obtained, reducedthermal resistance is obtained on the interfaces so that it is possibleto obtain a thermally conductive sheet having high thermal conductivityin the thickness direction, which is suitable for use as a member to beinterposed between any of various heat sources (for example, variousdevices, such as a CPU, a transistor, and an LED) and a heat radiatingmember.

Moreover, it is also found that, upon cutting the cured object of thethermally conductive composition into a given thickness with theultrasonic cutter, by slicing the cured object, with the cured object(thermally conductive sheet) to be cut with the ultrasonic cutter beingdisposed so that the anisotropic thermally conductive filler is orientedwith an angle of 5° to 45° relative to the thickness direction of thecured object to be cut with the ultrasonic cutter, the applied angleallows the anisotropic thermally conductive filler to easily fall whenthe sheet is pasted between semiconductor elements and a heat sink witha load being applied thereto (the anisotropic thermally conductivefiller is easily allowed to slide within the thermally conductivesheet), thereby making it possible to improve a compression rate whilesuppressing an increase in thermal resistance.

The present invention has been devised based upon the findings by thepresent inventors, and the following arrangements are prepared as themeans for solving the above-mentioned problems.

<1> A method for producing a thermally conductive sheet characterized byincluding at least an extrusion molding step of: by extruding athermally conductive composition containing a polymer, an anisotropicthermally conductive filler and a filler through an extruder, extrusionmolding an extrusion molded product in which the anisotropic thermallyconductive filler is oriented along the extrusion direction;

a curing step of curing the extrusion molded product to form a curedobject; and

a cutting step of cutting the cured object in a direction perpendicularto the extrusion direction into a given thickness with an ultrasoniccutter.

<2> A method for producing a thermally conductive sheet characterized byincluding at least an extrusion molding step of by extruding a thermallyconductive composition containing a polymer, an anisotropic thermallyconductive filler and a filler through an extruder, extrusion-molding anextrusion molded product in which the anisotropic thermally conductivefiller is oriented along the extrusion direction;

a curing step of curing the extrusion molded product to form a curedobject; and

a cutting step in which upon cutting the cured object into a giventhickness with an ultrasonic cutter, a slicing process is carried out onthe cured object, with the cured object being disposed so that theanisotropic thermally conductive filler is oriented with an angle of 5°to 45° relative to the thickness direction of the cured object to be cutwith the ultrasonic cutter.

<3> The method for producing a thermally conductive sheet described ineither <1> or <2> in which the anisotropic thermally conductive fillerhas an average fiber length of 100 μm or more.<4> The method for producing a thermally conductive sheet described inany one of <1> to <3> in which the anisotropic thermally conductivefiller is prepared as carbon fibers.<5> The method for producing a thermally conductive sheet described inany one of <1> to <4> in which the anisotropic thermally conductivefiller has a content of 16% by volume to 25% by volume in the thermallyconductive composition.<6> The method for producing a thermally conductive sheet described inany one of <1> to <5> in which the filler has an average particle sizein a range from 1 μm to 40 μm.<7> The method for producing a thermally conductive sheet described inany one of <1> to <6> in which the filler is prepared as sphericalalumina particles.<8> The method for producing a thermally conductive sheet described inany one of <1> to <7> in which the polymer is a silicone resin.<9> A thermally conductive sheet produced by the method for producing athermally conductive sheet described in any one of <1> to <8>.<10> The thermally conductive sheet described in <9> in which aperipheral portion of the thermally conductive sheet has a slightstickiness that is higher than that of the inside of the thermallyconductive sheet.<11> The thermally conductive sheet described in <9> or <10> in whichthe thermally conductive sheet has a cut surface having a surfaceroughness Ra of 9.9 μm or less.

Effects of Invention

The present invention makes it possible to solve the above-mentionedvarious conventional problems, and also to achieve the above-mentionedobject, and since cut surfaces with low surface roughness are obtained,reduced thermal resistance is obtained on the interfaces so that it ispossible to obtain a thermally conductive sheet having high thermalconductivity in the thickness direction, which is suitable for use as amember to be interposed between any of various heat sources and a heatradiating member, and a method for producing such a thermally conductivesheet.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a schematic diagram that shows a flow of a producing methodfor a thermal conductive sheet of the present invention.

FIG. 2 is an explanatory view that explains an oriented state of ananisotropic thermal conductive filler in an extrusion molding process.

FIG. 3 is a photograph that shows a state in which a silicone curedobject of embodiment 1 is cut with an ultrasonic cutter.

FIG. 4A is an electron microscopic photograph showing a surface of a cutface of the thermal conductive sheet of embodiment 1 that has been cutwith the ultrasonic cutter; FIG. 4B is an electron microscopicphotograph showing the cut face of the thermal conductive sheet ofembodiment 1 that has been cut with the ultrasonic cutter; and FIG. 4Cis a three-dimensional graphic view showing the cut face of the thermalconductive sheet of embodiment 1 that has been cut with the ultrasoniccutter.

FIG. 5A is an electron microscopic photograph showing a surface of a cutface of a thermal conductive sheet of comparative embodiment 1 that hasbeen cut with a commercial cutter knife; FIG. 5B is an electronmicroscopic photograph showing the cut face of the thermal conductivesheet of comparative embodiment 1 that has been cut with the commercialcutter knife; and FIG. 5C is a three-dimensional graphic view showingthe cut face of the thermal conductive sheet of comparative embodiment 1that has been cut with the commercial cutter knife.

FIG. 6 is a graph that shows a relationship between a thickness and athermal resistance obtained when cutting processes are carried out byusing the commercial cutter knife and the ultrasonic cutter, with theirthicknesses being changed.

FIG. 7 is a photograph of a cross section in the thickness direction ofthe thermal conductive sheet of embodiment 1.

FIG. 8 is a photograph of a cross section in the thickness direction ofthe thermal conductive sheet formed in accordance with embodiment 1described in JP-A No. 2003-200437.

FIG. 9 is a view for use in explaining an angle to be formed between anextrusion direction (length direction) of a cured object and a blade ofthe ultrasonic cutter.

FIG. 10 is a graph that shows a relationship among an angle, a thermalresistance and a compression rate of a carbon fiber relative to thethickness direction of a thermal conductive sheet of embodiment 16 uponapplication of a load of 1 kgf/cm².

FIG. 11 is a graph that shows a relationship among an angle, a thermalresistance and a compression rate of a carbon fiber relative to thethickness direction of the thermal conductive sheet of embodiment 16upon application of a load of 2 kgf/cm².

FIG. 12 is a graph that shows a relationship among an angle, a thermalresistance and a compression rate of a carbon fiber relative to thethickness direction of the thermal conductive sheet of embodiment 16upon application of a load of 3 kgf/cm².

FIG. 13 is a table showing a compression rate of each of a firstsilicone resin and a second silicone resin relative to their compoundingratio.

FIG. 14 is a table showing evaluations carried out on a burning test andeasiness in extrusion of a sheet base material.

FIG. 15 is a graph showing a relationship between a compounding amountof carbon fibers and a thermal resistance in the thermally conductivesheet.

FIG. 16 is a table showing compounding amounts of materials forming thethermally conductive sheet.

FIG. 17 is a perspective view that shows processes for producing athermally conductive sheet by slicing the sheet base material.

FIG. 18 is an outside view showing a slicing device.

FIG. 19 is a graph that shows a relationship between a slicing methodthat depends on the present or absence of ultrasonic vibrations and athermal resistance value of the thermally conductive sheet.

FIG. 20 is a view that shows a slicing speed of an ultrasonic cutter anda shape of the thermally conductive sheet in association with thethickness thereof.

FIG. 21 is a table that shows a slicing speed of the sheet base materialand characteristics of the thermally conductive sheet, which depend on adifference in thickness of the thermally conductive sheet.

FIG. 22 is a table that shows respective characteristics of thermallyconductive sheets that are sliced with the amplitude of ultrasonicvibrations to be applied to the cutter being changed.

DETAILED DESCRIPTION OF THE INVENTION (Thermally Conductive Sheet andProduction Method of Thermally Conductive Sheet)

The producing method of a thermally conductive sheet of the presentinvention includes at least an extrusion molding process, a curingprocess and a cutting process, and also includes other processes, ifnecessary.

The thermally conductive sheet of the present invention is produced bythe production method of the thermally conductive sheet of the presentinvention.

While giving an explanation of the producing method for the thermallyconductive sheet of the present invention, the thermally conductivesheet of the present invention will also be clearly described in detail.

As shown in FIG. 1, in accordance with the producing method, thethermally conductive sheet of the present invention is produced througha series of processes including an extruding process, a molding process,a curing process, a cutting (slicing) process and the like.

First, as shown in FIG. 1, a thermally conductive composition containinga polymer, an anisotropic thermally conductive filler and a filler isprepared. Next, when the thermally conductive composition thus preparedis extruded and molded, by allowing the composition to pass through aplurality of slits, the anisotropic thermally conductive filler blendedin the thermally conductive composition is oriented in the thicknessdirection of the thermally conductive sheet. After the resulting moldedproduct has been cured, the cured object 11 is cut in a directionperpendicular to the extrusion direction by using an ultrasonic cutterinto a given thickness; thus, since cut surfaces with low surfaceroughness are obtained, reduced thermal resistance is obtained on theinterfaces so that it is possible to produce a thermally conductivesheet having high thermal conductivity in the thickness direction.

Moreover, as shown in FIG. 9, a cured object 11 obtained by curing theresulting molded product is disposed so as to allow the extrusiondirection D of the cured object 11 to have a predetermined angle withthe blade of an ultrasonic cutter 14 (45° in FIG. 9A, 0° in FIG. 9B and90° in FIG. 9C) and is cut so as to have a given thickness so that whenpasted to be interpolated between the semiconductor element and the heatsink with a load applied thereto, by the angle prepared thereto, theanisotropic thermally conductive filler 1 is allowed to easily fall (theanisotropic thermally conductive filler is allowed to easily slidewithin the thermally conductive sheet); thus, it is possible to form athermally conductive film having an improved compression rate whilesuppressing an increase in thermal resistance. In this case, the anglemade by the extrusion direction D (length direction) of the cured object11 and the blade of the ultrasonic cutter 14 is the same as the orientedangle of the anisotropic thermally conductive filler 1 relative to thethickness direction of the thermally conductive sheet.

<Extrusion Molding Process>

The above-mentioned extrusion molding process is a process in which athermally conductive composition containing a polymer, an anisotropicthermally conductive filler and a filler is extruded by an extruder sothat an extrusion molded product in which the anisotropic thermallyconductive filler is oriented along the extrusion direction is produced.

—Polymer—

As the above-mentioned polymer, not particularly limited, any polymermay be selected on demand in accordance with performances required forthe thermally conductive sheet, and for example, a thermoplastic polymeror a thermosetting polymer may be used.

As the thermoplastic polymer, a thermoplastic resin, a thermoplasticelastomer or a polymer alloy formed by these may be used.

As the thermoplastic resin, not particularly limited, any resin may beselected on demand in accordance with objectives thereof, and examplesthereof include: an ethylene-α-olefin copolymer, such as polyethylene,polypropylene and an ethylene-propylene copolymer; fluorine-basedresins, such as polymethyl pentene, polyvinyl chloride, polyvinylidenechloride, polyvinyl acetate, ethylene-vinylacetate copolymer, polyvinylalcohol, polyacetal, polyvinylidene fluoride, polytetrafluoroethylene,etc.; polyethylene terephthalate, polybutylene terephthalate,polyethylene naphthalate, polystyrene, polyacrylonitrile, astyrene-acrylonitrile copolymer, an acrylonitrile-butadiene-styrenecopolymer (ABS) resin, polyphenylene ether, modified polyphenyleneether, aliphatic polyamides, aromatic polyamides, polyamideimide,polymethacrylic acid or esters thereof, polyacrylic acid or estersthereof, polycarbonate, polyphenylene sulfide, polysulfone, polyethersulfone, polyether nitrile, polyether ketone, polyketone, liquid crystalpolymer, silicone resin, ionomer, etc. One of these may be used alone,or two or more kinds of these may be used in combination.

Examples of the above-mentioned thermoplastic elastomer includestyrene-based thermoplastic elastomers, such as a styrene-butadienecopolymer or a hydrogenated polymer thereof, a styrene-isoprene blockcopolymer or a hydrogenated polymer thereof, olefin-based thermoplasticelastomers, vinylchloride-based thermoplastic elastomers,polyester-based thermoplastic elastomers, polyurethane-basedthermoplastic elastomers, polyamide-based thermoplastic elastomers, etc.One kind of these may be used alone, or two or more kinds of these maybe used in combination.

Examples of the thermosetting polymer include: cross-linking rubbers,epoxy resins, polyimide resins, bismaleimide resins, benzocyclobuteneresins, phenol resins, unsaturated polyesters, diallyl phthalate resins,silicone resins, polyurethane, polyimide silicone, thermosettingpolyphenylene ether, thermosetting denatured polyphenylene ether, etc.One of these may be used alone, or two or more kinds of these may beused in combination.

Examples of the above-mentioned cross-linking rubbers include: naturalrubbers, butadiene rubbers, isoprene rubbers, nitrile rubbers,hydrogenated nitrile rubbers, chloroprene rubbers, ethylene propylenerubbers, chlorinated polyethylene, chlorosulfonated polyethylene, butylrubbers, halogenated butyl rubbers, fluorine rubbers, urethane rubbers,acrylic rubbers, polyisobutylene rubbers, silicone rubbers, etc. One ofthese may be used alone, or two or more kinds of these may be used incombination.

Among these, the silicone resins are in particular preferably usedbecause of their superior molding processability and weatherability aswell as from the viewpoints of good adhesion and follow-up property toelectronic parts.

The above-mentioned silicone resin is not particularly limited, but isdesirably selected depending on its purposes, and examples thereofinclude: addition reaction type liquid silicone rubbers, siliconerubbers of a thermal vulcanization millable type using peroxide forvulcanizing process thereof, etc. Among these, since adhesion between aheat radiating surface and a heat sink surface of an electronic part isrequired as the heat radiating member of an electronic apparatus,addition reaction type liquid silicone rubbers are in particularpreferably used.

—Anisotropic Thermally Conductive Filler—

With respect to the shape of the anisotropic thermally conductivefiller, not particularly limited, selection can be made on demanddepending on purposes, and examples thereof include: a scale shape, aplate shape, a column shape, a rectangular pillar shape, an ellipticalshape, a flattened shape, etc. Among these, from the viewpoint ofanisotropic thermal conductivity, the flattened shape is preferablyused, in particular.

With respect to the above-mentioned filler having an anisotropicproperty, for example, boron nitride (BN) powder, graphite, carbonfibers are proposed. Among these, from the viewpoint of anisotropicthermal conductivity, carbon fibers are more preferably used.

With respect to the carbon fibers, for example, pitch-based carbonfibers, PAN-based carbon fibers, and those carbon fibers synthesized byusing an are discharge method, a laser evaporation method, a CVD method(Chemical Vapor Deposition Method), a CCVD method (Catalyst ChemicalVapor Deposition Method) and the like may be used. Among these, inparticular, the pitch-based carbon fibers are preferably used from theviewpoint of thermal conductivity.

One portion or the entire portion of the carbon fibers may be subjectedto a surface treatment, if necessary, and then used. As the surfacetreatment, for example, an oxidizing treatment, a nitriding treatment, anitrating treatment, sulfonating treatment, or a treatment in which onthe surface of a functional group introduced thereon by these treatmentsor carbon fibers, a metal, a metal compound, an organic compound or thelike is adhered or combined may be used. As the above-mentionedfunctional group, for example, a hydroxyl group, a carboxyl group, acarbonyl group, a nitro group, an amino group, or the like can beproposed.

The average major axis length (average fiber length) of theabove-mentioned carbon fibers is preferably set to 100 μm or more, morepreferably to 120 μm to 6 mm. In the case when the average major axislength is less than 100 μm, it is not possible to obtain a sufficientanisotropic thermal conductivity in some cases, with the result that ahigh thermal resistance is caused.

The average minor axis length of the above-mentioned carbon fibers ispreferably set to 6 μm to 15 μm, more preferably to 8 μm to 13 μm.

The aspect ratio of the above-mentioned carbon fibers (average majoraxis length/average minor axis length) is preferably set to 8 or more,more preferably, to 12 to 30. In the case when the aspect ratio is lessthan 8, the thermal conductivity tends to be lowered because of theshort fiber length (major axis length) of the carbon fibers.

In this case, the average major axis length and the average minor axislength of the carbon fibers can be measured by using, for example, amicroscope, a scanning electron microscope (SEM), or the like.

The content of the anisotropic thermal conductive filler in theabove-mentioned thermal conductive composition is preferably set to 15%by volume to 26% by volume. In the case of the content of less than 15%by volume, it is sometimes not possible to provide a sufficient thermalconductivity to the molded product, while in the case of the contentexceeding 26% by volume, adverse effects are sometimes given to themoldability and the orientation property.

—Filler—

With respect to the above-mentioned filler, the shape, material, averagegrain size, etc. thereof are not particularly limited, but can beselected on demand depending on purposes thereof. With respect to theshape, although not particularly limited, selection can be made ondemand depending on purposes thereof, and for example, a sphericalshape, an elliptic spherical shape, a blocky shape, a grain shape, aflattened shape, a needle shape, etc. may be used. Among these, from theviewpoint of filling property, the spherical shape and the ellipticspherical shape are preferably used, and the spherical shape is inparticular more preferably used.

With respect to the material for the filler, examples thereof include:aluminum nitride, silica, alumina, boron nitride, titania, glass, zincoxide, silicon carbide, silicon, silicon oxide, aluminum oxide, metalparticles, etc. One kind of these may be used alone, or two or morekinds of these may be used in combination. Among these, alumina, boronnitride, aluminum nitride, zinc oxide and silica are preferably used,and from the viewpoint of thermal conductivity, alumina and aluminumnitride are in particular preferably used.

In this case, the above-mentioned filler may be subjected to a surfacetreatment. When the surface treatment is carried out with a couplingagent, the dispersibility is improved, and the flexibility of thethermally conductive sheet is improved. Moreover, the surface roughnessderived from the slicing can be made smaller.

The average particle size of the filler is preferably set to 1 μm to 40μm, more preferably, to 1 μm to 20 μm. The average particle size of lessthan 1 μm might cause an insufficient curing process, and the averageparticle size exceeding 40 μm sometimes disturbs the orientation ofcarbon fibers to cause a reduction in the thermal conductivity of thecured object.

The average particle size of the filler can be measured by using, forexample, a grain size distribution meter and a scanning electronmicroscope (SEM).

The content of the filler in the above-mentioned thermally conductivecomposition is preferably set to 40% by volume to 60% by volume.

To the above-mentioned thermally conductive composition, if necessary,other components, such as a solvent, a thixotropy-adding agent, adispersant, a curing agent, a curing accelerator, a retarding agent, afine tackifier, a plasticizer, a flame retardant, an antioxidant, astabilizer, a colorant and the like, may be further added to be blendedtherein.

The above-mentioned thermally conductive composition can be prepared bymixing the polymer, the anisotropic thermally conductive filler and thefiller, as well as the other components, if necessary, with one anotherby using a mixer or the like.

Next, the thermally conductive composition is extruded into a mold 3 byusing a pump, an extruder or the like to be molded therein (see FIG. 1).A plurality of slits are formed on the extrusion outlet of the extruderso that the anisotropic thermally conductive filler is subsequentlyoriented in the extrusion direction.

The shape and size of the slits are not particularly limited, andselection may be made depending on purposes thereof, and for example, aflat plate shape, a lattice shape, a honeycomb shape and the like areproposed as the shape of the slits. As the size (width) of the slits,that in a range from 0.5 mm to 10 mm is preferably used.

The extruding rate of the thermally conductive composition is preferablyset to 0.001 L/min.

With respect to the mold 3, the shape, size, material and the like arenot particularly limited, and selection is made on demand depending onpurposes thereof; as the shape, for example, a hollow column shape, ahollow rectangular pillar shape and the like are proposed. The size canbe selected on demand in accordance with the size of the thermallyconductive sheet to be produced. As the material, for example, stainlessis proposed.

During a process in which the thermally conductive composition isallowed to pass through the extruder or the like, the anisotropicthermally conductive filler, the filler, etc. are collected in thecentral direction of the thermally conductive composition so that theanisotropic thermally conductive filler and the filler have differentdensities between the surface and the central portion. That is, since onthe surface of the thermally conductive composition (molded product)that has passed through the extruder, none of the thermally conductivefiller and anisotropic thermally conductive filler protrude onto thesurface, the surface portion (outer circumferential portion of thethermally conductive sheet) of the cured object 11 formed by curing thethermally conductive composition (molded product) has a preferableslight stickiness so that a preferable adhesive property to an adhesionobject (a semiconductor device or the like) is exerted. In contrast, onthe surface to be made in contact with a heat source or a heat radiatingside, since the anisotropic thermally conductive filler is protrudedtherefrom, its slight stickiness is lowered.

Moreover, as shown in FIG. 2, by extrusion molding the anisotropicthermally conductive filler 1 and the thermally conductive compositioncontaining the spherical filler 2, the fiber-state anisotropic thermallyconductive filler 1 can be oriented in the extrusion direction.

In this case, the slight stickiness refers to such a characteristic asto have a re-peeling property that is less susceptible to an increase inadhesive strength with the lapse of time and due to wet heat and also tohave a stickiness in such a degree that when pasted onto the adhesionobject, a positional deviation is not easily caused.

<Curing Process>

The above-mentioned curing process is a process by which theabove-mentioned extrusion molded product is cured to form a curedobject.

The molded product molded in the above-mentioned extrusion moldingprocess is formed into a complete cured object through a curing reactionappropriately adjusted depending on a resin to be used.

With respect to the curing methods of the extrusion molded product, notparticularly limited, selection is made on demand depending on purposesthereof, and in the case when a thermosetting resin such as a siliconeresin is used as a polymer, the curing process is preferably carried outby heating.

As the device for use in the above heating process, for example, a farinfrared furnace, a hot air furnace, etc. are proposed.

With respect to the heating temperature, not particularly limited,selection may be made on demand depending on purposes thereof, and forexample, a temperature range of 40° C. to 150° C. is preferably used.

With respect to the flexibility of the silicone cured object formed bycuring the silicone resin, not particularly limited, selection is madeon demand depending on purposes thereof, and it can be adjusted, forexample, by the crosslinking density of the silicone, the filling amountof the thermally conductive filler, etc.

<Cutting Process>

In a first mode, the cutting process is a process in which the curedobject is cut by using an ultrasonic cutter in a direction perpendicularto the extrusion direction into a given thickness.

In a second mode, the cutting process is a process in which, uponcutting the cured object into a given thickness by using an ultrasoniccutter, the cured object is disposed so as to allow the anisotropicthermally conductive filler to be oriented with an angle from 5° to 45°relative to the thickness direction of the cured object to be cut by theultrasonic cutter, and then cut.

Additionally, the ultrasonic cutter is secured so that the position ofthe blade of the ultrasonic cutter is unchanged.

The angle made by the thickness direction of the cured object (thermallyconductive sheet) to be cut into a given thickness with the ultrasoniccutter and the anisotropic thermally conductive filler is preferably setto 5° to 45°, more preferably to 5° to 30°. In the case when the anglethus made is less than 5°, the compression rate is unchanged from thatat the time of 0°, while in the case when it exceeds 45°, the thermalresistance value is sometimes increased.

The angle thus made can be measured by using, for example, an electronmicroscope.

The cutting process is carried out by using the ultrasonic cutter. Inthe ultrasonic cutter, the transmission frequency and the amplitude canbe adjusted, and the transmission frequency is preferably adjusted in arange from 10 kHz to 100 kHz, and the amplitude is preferably adjustedin a range from 10 μm to 100 μm. In the case when the cutting process iscarried out by using not the ultrasonic cutter, but a cutter knife or ameat slicer (rotary blade), the surface roughness Ra on the cut surfacebecomes larger to cause an increased thermal resistance.

In accordance with the cutting process in the first mode, by cutting thecured object in which the curing reaction has been completed in adirection perpendicular to the extrusion direction into a giventhickness by using the ultrasonic cutter, it becomes possible to obtaina thermally conductive sheet in which the anisotropic thermallyconductive filler (for example, carbon fibers, or scale shapedparticles) is oriented in the thickness direction (perpendicularlyoriented) of the thermally conductive sheet.

In accordance with the cutting process in the second mode, upon cuttingthe cured object into a given thickness by using the ultrasonic cutter,the cured object is disposed so as to allow the anisotropic thermallyconductive filler to be oriented with an angle from 5° to 45° relativeto the thickness direction of the cured object to be cut by theultrasonic cutter (thermally conductive sheet), and then cut; thus, theanisotropic thermally conductive filler inside the thermally conductivesheet is allowed to easily fall (the anisotropic thermally conductivefiller is allowed to easily slide within the thermally conductivesheet); thus, it is possible to improve a compression rate whilesuppressing an increase in thermal resistance.

The thickness of the thermally conductive sheet is preferably set to 0.1mm or more. When the thickness is less than 0.1 mm, the sheet sometimesfails to retain its shape when being sliced, depending on the hardnessof the cured object. There is a limitation in a method for orienting theanisotropic thermally conductive filler by applying a magnetic field toa thick sheet; however, the producing method of the thermally conductivesheet of the present invention is advantageous in that no limitation isgiven to the thickness of the sheet.

With respect to the thermally conductive sheet of the present invention,the oriented angle of the anisotropic thermally conductive filler (forexample, carbon fibers, or scale shaped particles) relative to thethickness direction of the thermally conductive sheet is preferably setto 0 degree to 45 degrees, more preferably, to 0 degree to 30 degrees.

The oriented angle of the carbon fibers can be measured by, for example,observing the cross section of the thermally conductive sheet with amicroscope.

With respect to the thermally conductive sheet produced by the producingmethod of the thermally conductive sheet of the present invention, thesurface roughness Ra of the cut surface after the cutting process ispreferably set to 9.9 μm or less, more preferably, to 9.5 μm or less. Inthe case when the surface roughness Ra exceeds 9.9 μm, the subsequentincreased surface roughness tends to cause a greater thermal resistance.

The above-mentioned surface roughness Ra can be measured by using, forexample, a laser microscope.

Since the thermally conductive sheet of the present invention is used asa member to be interposed between any of various heat sources (forexample, various devices, such as a CPU, a transistor and an LED) and aheat radiating member, it is preferably provided with a flame retardantproperty from the viewpoint of safety, and the flame retardant propertyis preferably set to “V-0” or more according to UL-94 standard.

—Application—

Since the thermally conductive sheet of the present invention has asmall surface roughness on a cut surface and a reduced thermalresistance on the interfaces with a high thermal conductivity in thethickness direction, it is suitable for use as a member to be interposedbetween any of various heat sources (for example, various devices, suchas a CPU, a transistor and an LED) and a heat radiating member; thus, itcan be desirably applied to peripheral parts of various electricdevices, such as, for example, a CPU, an MPU, a power transistor, anLED, a laser diode, various batteries (various secondary batteries, suchas lithium ion batteries, various fuel batteries, and various solarbatteries, such as wet-type solar batteries, including capacitors,amorphous silicon, crystal silicon, compound semiconductors, etc.),which are adversely influenced in efficiency of element operations,service life, etc., depending on, for example, temperatures; and theperipheral parts of heat sources of heating apparatuses and theperipheral parts of heating pipes of heat exchangers and floor heatingapparatuses in which heat needs to be effectively utilized.

EXAMPLES

The following description will discuss examples of the presentinvention; however, the present invention is not intended to be limitedby these examples.

In the following examples and comparative examples, the average particlediameters of alumina particles and aluminum nitride particles are valuesmeasured by using a grain size distribution meter. Moreover, the averagemajor axis length and the average minor axis length of pitch-basedcarbon fibers are values measured by a microscope (K117700, made byHiROX Co., Ltd.).

Example 1 Production of Thermally Conductive Sheet

To an addition reaction-type liquid-state silicone resin of a two-liquidtype prepared by mixing 18.8% by volume of a silicone A liquid(organopolysiloxane having a vinyl group) and 18.8% by volume of asilicone B liquid (organopolysiloxane having an H—Si group) were addedto be dispersed therein alumina particles (average particle size: 3 μm,Alumina DAW03, spherical shape, made by Denki Kagaku Kogyo KabushikiKaisha) (42.3% by volume) and pitch-based carbon fibers (average majoraxis length: 150 μm, average minor axis length: 8 μm, Raheama R-A301,made by Teijin Ltd.) (20.1% by volume) so that a silicone resincomposition was prepared.

The resulting silicone resin composition was extrusion-molded into amold 3 (hollow column shape) by using an extruder so that a siliconemolded product was formed. A slit (extrusion outlet shape: flat plate)was formed on the extrusion outlet of the extruder.

The resulting silicone molded product was heated at 100° C. for 1 hourin an oven so that a silicone cured object was prepared.

The resulting silicone cured object was cut into slices by an ultrasoniccutter so as to have a thickness of 0.5 mm (see FIG. 3, transmissionfrequency: 20.5 kHz, amplitude: 50 to 70 μm). As described above, athermally conductive sheet of example 1 having a square shape of 15 mmin longitudinal length and 15 mm in lateral length with a thickness of0.5 mm was prepared.

When the cross section of the resulting thermally conductive sheet wasobserved with a microscope (KH7700, made by HiROX Co., Ltd.), thepitch-based carbon fibers were oriented with an angle of 0 degree to 5degrees relative to the thickness direction of the thermally conductivesheet.

Example 2 Production of Thermally Conductive Sheet

The same processes as those of example 1 were carried out except that inexample 1, the alumina particles (average particle size: 3 μm, AluminaDAW03, spherical shape, made by Denki Kagaku Kogyo Kabushiki Kaisha)were changed to alumina particles (average particle size: 5 μm, AluminaDAW05, spherical shape, made by Denki Kagaku Kogyo Kabushiki Kaisha) sothat a thermally conductive sheet of example 2 having a square shape of15 mm in longitudinal length and 15 mm in lateral length with athickness of 0.5 mm was formed.

Example 3 Production of Thermally Conductive Sheet

The same processes as those of example 1 were carried out except that inexample 1, the alumina particles (average particle size: 3 μm, AluminaDAW03, spherical shape, made by Denki Kagaku Kogyo Kabushiki Kaisha)were changed to alumina particles (average particle size: 10 μm, AluminaDAW10, spherical shape, made by Denki Kagaku Kogyo Kabushiki Kaisha) sothat a thermally conductive sheet of example 3 having a square shape of15 mm in longitudinal length and 15 mm in lateral length with athickness of 0.5 mm was formed.

Example 4 Production of Thermally Conductive Sheet

The same processes as those of example 1 were carried out except that inexample 1, to an addition reaction-type liquid-state silicone resin of atwo-liquid type prepared by mixing 17.8% by volume of a silicone Aliquid (organopolysiloxane having a vinyl group) and 17.8% by volume ofa silicone B liquid (organopolysiloxane having an H—Si group) were addedto be dispersed therein alumina particles (average particle size: 3 μm,Alumina. DAW03, spherical shape, made by Denki Kagaku Kogyo KabushikiKaisha) (41.0% by volume) and pitch-based carbon fibers (average majoraxis length: 150 it in, average minor axis length: 8 μm, Raheama R-A301,made by Teijin Ltd.) (23.4% by volume) so that a silicone resincomposition was prepared; thus, a thermally conductive sheet of example4 having a square shape of 15 mm in longitudinal length and 15 mm inlateral length with a thickness of 0.5 mm was formed.

Example 5 Production of Thermally Conductive Sheet

The same processes as those of example 1 were carried out except that inexample 1, to an addition reaction-type liquid-state silicone resin of atwo-liquid type prepared by mixing 17.6% by volume of a silicone Aliquid (organopolysiloxane having a vinyl group) and 17.6% by volume ofa silicone B liquid (organopolysiloxane having an H—Si group) were addedto be dispersed therein alumina particles (average particle size: 3 μm,Alumina DAW03, made by Denki Kagaku Kogyo Kabushiki Kaisha) (40.5% byvolume) and pitch-based carbon fibers (average major axis length: 150μm, average minor axis length: 8 μm, Raheama R-A301, made by TeijinLtd.) (24.3% by volume) so that a silicone resin composition wasprepared; thus a thermally conductive sheet of example 5 having a squareshape of 15 mm in longitudinal length and 15 mm in lateral length with athickness of 0.5 mm was formed.

Example 6 Production of Thermally Conductive Sheet

The same processes as those of example 1 were carried out except that inexample 1, to an addition reaction-type liquid-state silicone resin of atwo-liquid type prepared by mixing 19.5% by volume of a silicone Aliquid (organopolysiloxane having a vinyl group) and 19.5% by volume ofa silicone B liquid (organopolysiloxane having an H—Si group) were addedto be dispersed therein alumina particles (average particle size: 3 μm,Alumina DAW03, spherical shape, made by Denki Kagaku Kogyo KabushikiKaisha) (45.0% by volume) and pitch-based carbon fibers (average majoraxis length: 150 μm, average minor axis length: 8 μm, Raheama R-A301,made by Teijin Ltd.) (16.0% by volume) so that a silicone resincomposition was prepared; thus a thermally conductive sheet of example 6having a square shape of 15 mm in longitudinal length and 15 mm inlateral length with a thickness of 0.5 mm was formed.

Example 7 Production of Thermally Conductive Sheet

The same processes as those of example 1 were carried out except that inexample 1, to an addition reaction-type liquid-state silicone resin of atwo-liquid type prepared by mixing 18.9% by volume of a silicone Aliquid (organopolysiloxane having a vinyl group) and 18.9% by volume ofa silicone B liquid (organopolysiloxane having an H—Si group) were addedto be dispersed therein alumina particles (average particle size: 3 μm,Alumina DAW03, spherical shape, made by Denki Kagaku Kogyo KabushikiKaisha) (43.6% by volume) and pitch-based carbon fibers (average majoraxis length: 150 μm, average minor axis length: 8 μm, Raheama R-A301,made by Teijin Ltd.) (18.6% by volume) so that a silicone resincomposition was prepared; thus a thermally conductive sheet of example 7having a square shape of 15 mm in longitudinal length and 15 mm inlateral length with a thickness of 0.5 mm was formed.

Example 8 Production of Thermally Conductive Sheet

The same processes as those of example 1 were carried out except that inexample 1, the outer peripheral portion of a produced thermallyconductive sheet was cut with a commercial cutter knife so that athermally conductive sheet of example 8 having a square shape of 14 mmin longitudinal length and 14 mm in lateral length with a thickness of0.5 mm was formed.

Example 9 Production of Thermally Conductive Sheet

The same processes as those of example 1 were carried out except that inexample 1, the pitch-based carbon fibers (average major axis length: 150μm, average minor axis length: 8 μm, Raheama R-A301, made by TeijinLtd.) was changed to pitch-based carbon fibers (average major axislength: 100 μm, average minor axis length: 8 μm, Raheama R-A401, made byTeijin Ltd.) so that a thermally conductive sheet of example 9 having asquare shape of 15 mm in longitudinal length and 15 mm in lateral lengthwith a thickness of 0.5 mm was formed.

Example 10 Production of Thermally Conductive Sheet

The same processes as those of example 1 were carried out except that inexample 1, the pitch-based carbon fibers (average major axis length: 150μm, average minor axis length: 8 μm, Raheama R-A301, made by TeijinLtd.) was changed to pitch-based carbon fibers (average major axislength: 50 μm, average minor axis length: 8 μm, Raheama R-A201, made byTeijin Ltd.) so that a thermally conductive sheet of example 10 having asquare shape of 15 mm in longitudinal length and 15 mm in lateral lengthwith a thickness of 0.5 mm was formed.

Example 11 Production of Thermally Conductive Sheet

The same processes as those of example 1 were carried out except that inexample 1, to an addition reaction-type liquid-state silicone resin of atwo-liquid type prepared by mixing 17.3% by volume of a silicone Aliquid (organopolysiloxane having a vinyl group) and 17.3% by volume ofa silicone B liquid (organopolysiloxane having an H—Si group) were addedto be dispersed therein alumina particles (average particle size: 3 μm,Alumina DAW03, spherical shape, made by Denki Kagaku Kogyo KabushikiKaisha) (39.9% by volume) and pitch-based carbon fibers (average majoraxis length: 150 μm, average minor axis length: 8 μm, Raheama R-A301,made by Teijin Ltd.) (25.5% by volume) so that a silicone resincomposition was prepared; thus a thermally conductive sheet of example11 having a square shape of 15 mm in longitudinal length and 15 mm inlateral length with a thickness of 0.5 mm was formed.

Example 12 Production of Thermally Conductive Sheet

The same processes as those of example 1 were carried out except that inexample 1, the alumina particles (average particle size: 3 μm, AluminaDAW03, spherical shape, made by Denki Kagaku Kogyo Kabushiki Kaisha)were changed to alumina particles (average particle size: 45 μm, AluminaDAW45, spherical shape, made by Denki Kagaku Kogyo Kabushiki Kaisha) sothat a thermally conductive sheet of example 12 having a square shape of15 mm in longitudinal length and 15 mm in lateral length with athickness of 0.5 mm was formed.

Example 13 Production of Thermally Conductive Sheet

The same processes as those of example 1 were carried out except that inexample 1, the alumina particles (average particle size: 3 μm, AluminaDAW03, spherical shape, made by Denki Kagaku Kogyo Kabushiki Kaisha)(42.3 parts by mass) were changed to alumina particles (average particlesize: 3 μm, Alumina DAW03, spherical shape, made by Denki Kagaku KogyoKabushiki Kaisha) (25 parts by mass) and aluminum nitride (averageparticle size: 1 μm, made by Tokuyama Corporation) (17.3 parts by mass)so that a thermally conductive sheet of example 13 having a square shapeof 15 mm in longitudinal length and 15 mm in lateral length with athickness of 0.5 mm was formed.

Comparative Example 1 Production of Thermally Conductive Sheet

The same processes as those of example 1 were carried out except that inexample 1, the produced silicone cured object was cut into slices with acommercial cutter knife so as to have a thickness of 0.5 mm so that athermally conductive sheet of comparative example 1 having a squareshape of 15 mm in longitudinal length and 15 mm in lateral length with athickness of 0.5 mm was formed.

In this case, FIG. 4A is an electron microscopic photograph showing asurface of a cut face of the thermally conductive sheet of example 1,FIG. 4B is an electron microscopic photograph showing a cross sectionthereof, and FIG. 4C is a three-dimensional graphic view thereof,respectively.

Moreover, FIG. 5A is an electron microscopic photograph showing asurface of a cut face of the thermally conductive sheet of comparativeexample 1, FIG. 5B is an electron microscopic photograph showing a crosssection thereof, and FIG. 5C is a three-dimensional graphic viewthereof, respectively.

When the slice cutting process was carried out by using an ultrasoniccutter as shown in example 1, it was found that the surface roughnessbecame smaller with a reduced thermal resistance in comparison with thatin the case of slice cutting process by the use of a commercial cutterknife as shown in comparative example 1.

Comparative Example 2 Production of Thermally Conductive Sheet

The same processes as those of example 1 were carried out except that inexample 1, the produced silicone cured object was cut into slices havinga thickness of 0.5 with a meat slicer (rotary blade) (Remacom ElectricSlicer RSL-A19) so that a thermally conductive sheet of comparativeexample 2 having a square shape of 15 mm in longitudinal length and 15mm in lateral length with a thickness of 0.5 mm was formed.

Comparative Example 3 Production of Thermally Conductive Sheet

The same processes as those of example 1 were carried out except that inexample 1, the resulting silicone resin composition was stacked andcoated so as to produce a silicone stacked product, that this siliconestacked product was heated at 100° C. for 1 hour in an oven so that asilicone cured object was prepared and that the resulting silicone curedobject was cut into slices by an ultrasonic cutter so as to have athickness of 0.5 mm; thus, a thermally conductive sheet of comparativeexample 3 having a square shape of 15 mm in longitudinal length and 15mm in lateral length with a thickness of 0.5 mm was formed.

Comparative Example 4 Production of Thermally Conductive Sheet

The same processes as those of comparative example 3 were carried outexcept that in comparative example 3, the resulting silicone stackedproduct was cut into slices by using a commercial cutter knife in placeof the ultrasonic cutter so that a thermally conductive sheet ofcomparative example 4 having a square shape of 15 mm in longitudinallength and 15 mm in lateral length with a thickness of 0.5 mm wasformed.

Comparative Example 5 Production of Thermally Conductive Sheet

The same processes as those of comparative example 3 were carried outexcept that in comparative example 3, the resulting silicone stackedproduct was cut into slices by using a meat slicer (rotary blade)(Remacom Electric Slicer RSL-A19) in place of the ultrasonic cutter sothat a thermally conductive sheet of comparative example 5 having asquare shape of 15 mm in longitudinal length and 15 mm in lateral lengthwith a thickness of 0.5 mm was formed.

Next, with respect to the thermally conductive sheets of examples 1 to13 and comparative examples 1 to 5, various characteristics wereevaluated in the following manner. The results are shown in Table 1.

<Flame Retardant Property>

Flame retardant tests in accordance with UL-94 standard were carried outon each of the thermally conductive sheets so that the flame retardantproperty thereof was evaluated.

That is, test pieces indicated by UL94 were prepared, and based upon avertical burning test method of UL94V, a burning test was carried out oneach of the resulting test pieces. In this case, the burning time wasgiven as an addition of igniting periods of time of two times, and anaverage of 5 test pieces. The results thus obtained were evaluated asany one of grades of UL94 “V-0”, “V-1” and “V-2” in accordance with thefollowing criteria. Additionally, those did not satisfy any of thesewere evaluated as “disqualified”.

Based upon the vertical burning test method of UL94V, a burning test wascarried out on each of the resulting test pieces. In this case, theburning time was given as an addition of igniting periods of time of twotimes and an average of 5 test pieces. The results thus obtained wereevaluated as any one of grades of UL94 “V-0”, “V-1” and “V-2” inaccordance with the following criteria. Additionally, those did notsatisfy any of these were evaluated as “disqualified”.

(Evaluation Criteria)

“V-0”: An average burning time after removing an igniting flame was 10seconds or less, and none of all the samples dropped such a fine flameas to ignite cotton wool.

“V-1”: An average burning time after removing an igniting flame was 30seconds or less, and none of all the samples dropped such a fine flameas to ignite cotton wool.

“V-2”: An average burning time after removing an igniting flame was 30seconds or less, and the samples dropped such a fine flame as to ignitecotton wool.

<Surface Roughness Ra>

The surface roughness Ra of each of the thermally conductive sheets wasmeasured with a laser microscope.

<Slight Stickiness of Outer Peripheral Portion>

Each of the thermally conductive sheets was placed on a plastic plate ina direction perpendicular to the oriented direction of carbon fibers,and the slight stickiness of the outer peripheral portion was confirmed.

<Initial Thickness (Thickness Immediately After Cutting Process)>

The initial thickness of each of the thermally conductive sheets wasmeasured by using a thermal conductivity measuring device.

<Thermal Resistance>

The thermal resistance of each of the thermally conductive sheets wasmeasured by using a thermal conductivity measuring device in accordancewith ASTM D 5470, with a load of 1 kgf/cm² being applied thereto.

<Peeling Between Slits or on Stacked Surface>

With respect to the respective thermally conductive sheets, the presenceor absence of peeling between slits or on the stacked surface wasvisually confirmed.

TABLE 1 Components (% by volume) Example 1 Example 2 Example 3 Example 4Silicone A liquid 18.8 18.8 18.8 17.8 Silicone B liquid 18.8 18.8 18.817.8 Pitch-based carbon fibers (Raheama A301, 20.1 20.1 20.1 23.4average major-axis length 150 μm, average minor-axis length 8 μm, madeby Teijin Ltd.) Pitch-based carbon fibers (Raheama X401, — — — — averagemajor-axis length 100 μm, average minor-axis length 8 μm, made by TeijinLtd.) Pitch-based carbon fibers (Raheama A201, — — — — averagemajor-axis length 50 μm, average minor-axis length 8 μm, made by TeijinLtd.) Alumina DAW03 (average particle size 3 μm, 42.3 — — 41.0 made byDenki Kagaku Kogyo Kabushiki Kaisha) Alumina DAW05 (average particlesize 5 μm, — 42.3 — — made by Denki Kagaku Kogyo Kabushiki Kaisha)Alumina DAW10 (average particle size 10 μm, — — 42.3 — made by DenkiKagaku Kogyo Kabushiki Kaisha) Alumina DAW45 (average particle size 45μm, — — — — made by Denki Kagaku Kogyo Kabushiki Kaisha) Total (% byvolume) 100.0  100.0  100.0  100.0  Producing method Extrusion ExtrusionExtrusion Extrusion Slicing method Ultrasonic slice Ultrasonic sliceUltrasonic slice Ultrasonic slice Flame retardant property V-0 V-0 V-0V-0 Surface roughness Ra 3.7 μm 4.3 μm 9.9 μm 3.2 μm Slight stickinesson outer peripheral portion Yes Yes Yes Yes Initial thickness 0.5 mm 0.5mm 0.5 mm 0.5 mm Thermal resistance 0.15 K/W 0.14 K/W 0.22 K/W 0.13 K/WPealing between slits or on lamination surface No No No No Components (%by volume) Example 5 Example 6 Example 7 Example 8 Silicone A liquid17.6 19.5 18.9 18.8 Silicone B liquid 17.6 19.5 18.9 18.8 Pitch-basedcarbon fibers (Raheama A301, 24.3 16.0 18.6 20.1 average major-axislength 150 μm, average minor-axis length 8 μm, made by Teijin Ltd.)Pitch-based carbon fibers (Raheama X401, — — — — average major-axislength 100 μm, average minor-axis length 8 μm, made by Teijin Ltd.)Pitch-based carbon fibers (Raheama A201, — — — — average major-axislength 50 μm, average minor-axis length 8 μm, made by Teijin Ltd.)Alumina DAW03 (average particle size 3 μm, 40.5 45.0 43.6 42.3 made byDenki Kagaku Kogyo Kabushiki Kaisha) Alumina DAW05 (average particlesize 5 μm, — — — — made by Denki Kagaku Kogyo Kabushiki Kaisha) AluminaDAW10 (average particle size 10 μm, — — — — made by Denki Kagaku KogyoKabushiki Kaisha) Alumina DAW45 (average particle size 45 μm, — — — —made by Denki Kagaku Kogyo Kabushiki Kaisha) Total (% by volume) 100.0 100.0  100.0  100.0  Producing method Extrusion Extrusion ExtrusionExtrusion Slicing method Ultrasonic slice Ultrasonic slice Ultrasonicslice Ultrasonic slice Flame retardant property V-0 V-0 V-0 V-0 Surfaceroughness Ra 3.3 μm 6.6 μm 5.7 μm 8.2 μm Slight stickiness on outerperipheral portion Yes Yes Yes No Initial thickness 0.5 mm 0.5 mm 0.5 mm0.5 mm Thermal resistance 0.12 K/W 0.23 K/W 0.19 K/W 0.16 K/W Pealingbetween slits or on lamination surface No No No No Components (% byvolume) Example 9 Example 10 Example 11 Example 12 Example 13 Silicone Aliquid 18.8 18.8 17.3 18.8 18.8 Silicone B liquid 18.8 18.8 17.3 18.818.8 Pitch-based carbon fibers (Raheama A301, average — — 25.5 20.1 20.1major-axis length 150 μm, average minor-axis length 8 μm, made by TeijinLtd.) Pitch-based carbon fibers (Raheama X401, average 20.1 — — — —major-axis length 100 μm, average minor-axis length 8 μm, made by TeijinLtd.) Pitch-based carbon fibers Raheama A201, average — 20.1 — — —major-axis length 50 μm, average minor-axis length 8 μm, made by TeijinLtd.) Alumina DAW03 (average particle size 3 μm, 42.3 42.3 39.9 — 25made by Denki Kagaku Kogyo Kabushiki Alumina DAW05 (average particlesize 5 μm, — — — — — made by Denki Kagaku Kogyo Kabushiki Alumina DAW10(average particle size 10 μm, — — — — — made by Denki Kagaku KogyoKabushiki Alumina DAW45 (average particle size 45 μm, — — — 42.3 — madeby Denki Kegaku Kogyo Kabushiki Aluminum nitride (average particle size:1 μm, — — — — 17.3 made by Tokuyama Corporation) Total (% by volume)100.0  100.0  100.0  100.0  100.0  Producing method Extrusion ExtrusionExtrusion Extrusion Extrusion Slicing method Ultrasonic UltrasonicUltrasonic Ultrasonic Ultrasonic slice slice slice slice slice Flameretardant property V-0 V-0 V-0 V-0 V-0 Surface roughness Ra 7.8 μm 9.3μm 11.1 μm 9.6 μm 3.5 μm Slight stickiness on outer peripheral portionYes Yes Yes Yes Yes Initial thickness 0.5 mm 0.5 mm 0.5 mm 0.5 mm 0.5 mmThermal resistance 0.25 K/W 0.29 K/W 0.27 K/W 0.34 K/W 0.13 K/W Pealingbetween slits or on lamination surface No No Slightly present No NoComparative Comparative Comparative Comparative Comparative Components(% by volume) Example Example Example Example Example Silicone A liquid18.8 18.8 18.8 18.8 18.8 Silicone B liquid 18.8 18.8 18.8 18.8 18.8Pitch-based carbon fibers (Raheama A301, average 20.1 20.1 20.1 20.120.1 major-axis length 150 μm, average minor-axis length 8 μm, made byTeijin Ltd.) Pitch-based carbon fibers (Raheama X401, average — — — — —major-axis length 100 μm, average minor-axis length 8 μm, made by TeijinLtd.) Pitch-based carbon fibers (Raheama A201, average — — — — —major-axis length 50 μm, average minor-axis length 8 μm, made by TeijinLtd.) Alumina DAW03 (average particle size 3 μm, 42.3 42.3 42.3 42.342.3 made by Denki Kagaku Kogyo Kabushiki Kaisha) Alumina DAW05 (averageparticle size 5 μm, — — — — — made by Denki Kagaku Kogyo KabushikiKaisha) Alumina DAW10 (average particle size 10 μm, — — — — — made byDenki Kagaku Kogyo Kabushiki Kaisha) Alumina DAW45 (average particlesize 45 μm, — — — — — made by Denki Kagaku Kogyo Kabushiki Kaisha) Total(% by volume) 100.0  100.0  100.0  100.0  100.0  Producing methodExtrusion Extrusion Laminated Laminated Laminated coat coat coat Slicingmethod Cutter knife Meat slicer Ultrasonic Cutter knife Meat slicer(Rotary cutter (Rotary blade) blade) Flame retardant property V-1 V-1V-0 V-0 V-0 Surface roughness Ra 20.1 μm 23.2 μm 9.2 μm 20.8 μm 18.2 μmSlight stickiness on outer peripheral portion Yes Yes No No No Initialthickness 0.5 mm 0.5 mm 0.5 mm 0.5 mm 0.5 mm Thermal resistance 0.39 K/W0.37 K/W 0.17 K/W 0.37 K/W 0.33 K/W Pealing between slits or onlamination No No Yes Yes Yes surface

The results of Table 1 indicate that in the case when a slice cuttingprocess was carried out by using an ultrasonic cutter as shown inexamples 1 to 13, the thermal resistance was lowered in comparison withthe case where the slice cutting process was carried out by using acommercial cutter knife as shown in comparative example 1 so that adesirable thermal conductivity was exerted.

In example 9, the thermal resistance became slightly greater because ofits shorter fiber length of the pitch-based carbon fibers in comparisonwith that of example 1.

In example 10, the thermal resistance became slightly greater because ofits shorter fiber length of the pitch-based carbon fibers in comparisonwith that of example 1.

In example 11, since the filling amount of the pitch-based carbon fiberswas greater, and since the dispersion of the pitch-based carbon fiberswas slightly poor, in comparison with those of example 1, a slightlypeeled state on the interface was left even after the passage throughthe slit.

In example 12, since the average particle size was larger than that ofexample 1 to cause disturbance in the orientation of the pitch-basedcarbon fibers, the thermal resistance became slightly greater.

In comparative example 1, since a slice cutting process was carried outby using a commercial cutter in comparison with example 1, the surfaceirregularities became greater to cause an increased thermal resistance.

In comparative example 2, since a slice cutting process was carried outby using a meat slicer (rotary blade) in comparison with example 1, thesurface irregularities became greater to cause an increased thermalresistance.

In comparative example 3, since a lamination coated product was used incomparison with example 1, peeling occurred on the interface uponapplication of a load thereto. Moreover, since the lamination coatedproduct was not extruded into a mold, slight stickiness was not exertedon the outer peripheral portion.

In comparative example 4, since a lamination coated product was used incomparison with example 1, peeling occurred on the interface uponapplication of a load thereto. Moreover, since the lamination coatedproduct was not extruded into a mold, slight stickiness was not exertedon the outer peripheral portion. Since the slicing process was carriedout by using a commercial cutter knife, the surface irregularitiesbecame greater to cause an increased thermal resistance.

In comparative example 5, since a lamination coated product was used incomparison with example 1, peeling occurred on the interface uponapplication of a load thereto. Moreover, since the lamination coatedproduct was not extruded into a mold, slight stickiness was not exertedon the outer peripheral portion. Since the slicing process was carriedout by using a meat slicer (rotary blade), the surface irregularitiesbecame greater to cause an increased thermal resistance.

Example 14

The same processes as those of example 1 were carried out except that inexample 1, the silicone cured object was sliced with an ultrasoniccutter so as to have a thickness of 1.0 mm so that a thermallyconductive sheet of example 14 was formed.

When the resulting thermally conductive sheet was measured by applying aload of 1 kgf/cm², the resulting thickness was 0.9 mm.

Example 15

The same processes as those of example 1 were carried out except that inexample 1, the silicone cured object was sliced with an ultrasoniccutter so as to have a thickness of 1.5 mm so that a thermallyconductive sheet of example 15 was formed.

When the resulting thermally conductive sheet was measured by applying aload of 1 kgf/cm², the resulting thickness was 1.4 mm.

Comparative Example 6

The same processes as those of comparative example 1 were carried outexcept that in comparative example 1, the silicone cured object wassliced with a commercial cutter knife so as to have a thickness of 1.0mm so that a thermally conductive sheet of comparative example 6 wasformed.

When the resulting thermally conductive sheet was measured by applying aload of 1 kgf/cm² thereto, the resulting thickness was 0.9 mm.

Comparative Example 7

The same processes as those of comparative example 1 were carried outexcept that in comparative example 1, the silicone cured object wassliced with a commercial cutter knife so as to have a thickness of 1.5mm so that a thermally conductive sheet of comparative example 7 wasformed.

When the resulting thermally conductive sheet was measured by applying aload of 1 kgf/cm² thereto, the resulting thickness was 1.4 mm.

Next, with respect to example 1, examples 14 and 15, comparative example1 and comparative examples 6 and 7, the same processes as those ofexample 1 and comparative example 1 were carried out, with a load of 1kgf/cm² being applied thereto, so that the thermal resistance of each ofthe thermally conductive sheets was measured. In this case, when thethermally conductive sheet (0.5 mm in thickness immediately after thecutting process) of each of example 1 and comparative example 1 wasmeasured by applying a load of 1 kgf/cm² thereto, the resultingthickness was 0.4 mm. The results thereof are shown in FIG. 6.

The results shown FIG. 6 indicate that in example 1 and examples 14 and15 in which the ultrasonic cutter was used, the thermal resistance waslowered regardless of the thickness of the sheet in comparison withcomparative example 1 and comparative examples 6 and 7 in which thecommercial cutter knife was used, thereby making it possible to exert asuperior thermal conductivity.

Comparative Example 8

In the same manner as in example 1 described in JP-A No. 2003-200437,with a magnetic field being applied thereto at a normal temperature,graphitized carbon fibers, subjected to a surface treatment with asilane coupling agent, were oriented in a fixed direction by themagnetic field, and the carbon fibers were then thermally cured so thata thermally conductive sheet having a thickness of 2 mm of comparativeexample 8 was formed.

FIG. 8 shows a microscopic photograph (200 times in magnification) of across section in the thickness direction of the resulting thermallyconductive sheet of comparative example 8. Moreover, FIG. 7 shows amicroscopic photograph (200 times in magnification) of a cross sectionin the thickness direction of the thermally conductive sheet of example1.

The results shown in FIGS. 7 and 8 indicate that since, in comparativeexample 8, all the carbon fibers were oriented in the thicknessdirection (perpendicular direction) of the sheet as shown in FIG. 8, aproblem was raised in that when the sheet was bent, it was easily broken(cracked). In contrast, in example 1, in the case when the sheet wasproduced by an extrusion method as shown in FIG. 7, since some portionsof the carbon fibers were not oriented in the thickness direction(perpendicular direction), the sheet was hardly broken (hardly cracked)even when it was bent.

Example 16

A silicone cured object produced by using the same silicone resincomposition as that of example 1 was sliced with an ultrasonic cutter(transmission frequency: 20.5 kHz, amplitude: 50 to 70 μm) so as to havea thickness of 0.8 mm so that a thermally conductive sheet was produced.At this time, as shown in Table 2, the silicone cured objects weredisposed, with an angle, made by the thickness direction of the siliconecured object (thermally conductive sheet) cut with the ultrasonic cutterand the anisotropic thermally conductive filler (carbon fibers), beingvaried step by step from 0° to 90°, and ultrasonic sliced so thatthermally conductive sheets of samples No. 1 to No. 9 were produced.

With respect to the resulting thermally conductive sheets of samples No.1 to No. 9, various characteristics thereof in the case when a load of 1kgf/cm², a load of 2 kgf/cm² and a load of 3 kgf/cm² were respectivelyapplied thereto were measured. The results are shown in Table 2 andFIGS. 10 to 12.

The thermal resistance was measured in the same manner as in theaforementioned examples. Moreover, the compression rate was measured inthe following manner. The other physical properties were measured in thesame manner as in the aforementioned examples.

<Compression Rate>

The compression rate refers to a value (%) that indicates how muchdegree of thickness of a thermally conductive sheet is compressed uponapplication of a load thereto relative to the thickness of the thermallyconductive sheet prior to the measurement.

TABLE 2-1 Sample No. 1 2 3 4 5 Silicone A liquid 18.8 18.8 18.8 18.818.8 Silicone B liquid 18.8 18.8 18.8 18.8 18.8 Pitch-based carbonfibers (Raheama A301, 20.1 20.1 20.1 20.1 20.1 average major-axis length150 μm, average minor-axis length 8 μm, made by Teijin Ltd.) Pitch-basedcarbon fibers (Raheama X401, — — — — — average major-axis length 100 μm,average minor-axis length 8 μm, made by Teijin Ltd.) Pitch-based carbonfibers (Raheama A201, — — — — — average major-axis length 100 μm,average minor-axis length 8 μm, made by Teijin Ltd.) Alumina DAW03(DENKA Kabushiki Kaisha) 42.3 42.3 42.3 42.3 42.3 Alumina DAW05 (DENKAKabushiki Kaisha) — — — — — Alumina DAW10 (DENKA Kabushiki Kaisha) — — —— — Alumina DAW15 (DENKA Kabushiki Kaisha) — — — — —

minum nitride (average particle size: 1 μm, made by — — — — — TokuyamaCorporati

Total (% by volume) 100 100 100 100 100 Producing method ExtrusionExtrusion Extrusion Extrusion Extrusion Slicing method UltrasonicUltrasonic Ultrasonic Ultrasonic Ultrasonic slice slice slice sliceslice Load: Angle (°) of carbon fibers relative to thickness 0 5 10 1530 1 kgf/cm² direction of thermally conductive sheet Thermal resistance(K/W) 0.19 0.20 0.21 0.20 0.22 Compression rate (%) 4.5 5.6 6.8 7.8 8.5Load: Angle (°) of carbon fibers relative to thickness 0 5 10 15 30 2kgf/cm² direction of thermally conductive sheet Thermal resistance (K/W)0.15 0.16 0.16 0.17 0.19 Compression rate (%) 12.4 14.6 16.7 18.7 21.4Load: Angle (°) of carbon fibers relative to thickness 0 5 10 15 30 3kgf/cm² direction of thermally conductive sheet Thermal resistance (K/W)0.14 0.14 0.15 0.16 0.18 Compression rate (%) 22.5 24.6 26.8 28.8 31.4Flame retardant property V-0 V-0 V-0 V-0 V-0 Initial thickness 0.8 mm0.8 mm 0.8 mm 0.8 mm 0.8 mm Degree of orientation of carbon fiberswithin 0° 5° 10° 15° 30° Pealing between slits or on lamination surfaceNo No No No No Surface roughness 6.7 μm  7.2 μm  7.4 μm  7.9 μm  7.6 μm Sample No. 6 7 8 9 Silicone A liquid 18.8 18.8 18.8 18.8 Silicone Bliquid 18.8 18.8 18.8 18.8 Pitch-based carbon fibers (Raheama A301, 20.120.1 20.1 20.1 average major-axis length 150 μm, average minor-axislength 8 μm, made by Teijin Ltd.) Pitch-based carbon fibers (RaheamaX401, — — — — average major-axis length 100 μm, average minor-axislength 8 μm, made by Teijin Ltd.) Pitch-based carbon fibers (RaheamaA201, — — — — average major-axis length 100 μm, average minor-axislength 8 μm, made by Teijin Ltd.) Alumina DAW03 (DENKA Kabushiki Kaisha)42.3 42.3 42.3 42.3 Alumina DAW05 (DENKA Kabushiki Kaisha) — — — —Alumina DAW10 (DENKA Kabushiki Kaisha) — — — — Alumina DAW15 (DENKAKabushiki Kaisha) — — — —

inum nitride (average particle size: 1 μm, made by — — — — TokuyamaCorpora

Total (% by volume) 100 100 100 100 Producing method Extrusion ExtrusionExtrusion Extrusion Slicing method Ultrasonic Ultrasonic UltrasonicUltrasonic slice slice slice slice Load: Angle (°) of carbon fibersrelative to thickness 45 60 75 90 1 kgf/cm² direction of thermallyconductive sheet Thermal resistance (K/W) 0.28 0.4 0.67 0.79 Compressionrate (%) 13.64 9.95 11.057 12.06 Load: Angle (°) of carbon fibersrelative to thickness 45 60 75 90 2 kgf/cm² direction of thermallyconductive sheet Thermal resistance (K/W) 0.25 0.36 0.60 0.70Compression rate (%) 27.24 23.29 22.27 22.54 Load: Angle (°) of carbonfibers relative to thickness 45 60 75 90 3 kgf/cm² direction ofthermally conductive sheet Thermal resistance (K/W) 0.25 0.34 0.56 0.65Compression rate (%) 34.71 31.19 29.26 29.88 Flame retardant propertyV-0 V-0 V-0 V-0 Initial thickness 0.8 mm 0.8 mm 0.8 mm 0.8 mm Degree oforientation of carbon fibers within 45° 60° 75° 90° Pealing betweenslits or on lamination surface No No No No Surface roughness 8.1 μm  9.8μm  9.6 μm  8.4 μm 

indicates data missing or illegible when filed

The results shown in Table 2 and FIGS. 10 to 12 indicate that until theangle made by the thickness direction of the thermally conductive sheetand the anisotropic thermally conductive filler (carbon fibers) hasreached 45°, the compression rate increases in response to the load;however, when the angle made by the thickness direction of the thermallyconductive sheet and the carbon fibers exceeds 45°, the compression ratetends to be lowered reversely. Moreover, it is also found that thethermal resistance value abruptly deteriorates when the angle made bythe thickness direction of the thermally conductive sheet and the carbonfibers exceeds 45°.

Modified Example

By the way, since the thermally conductive sheet needs to have highflexibility and shape flow-up property, it is necessary to preventdeformation of the cured object serving as a sheet base material andalso to slice it into a thin uniform thickness. Moreover, in the casewhen the sliced surface of the thermally conductive sheet is rubbed withthe cutting blade by the frictional resistance to cause disturbance inthe orientation of the carbon fibers, a reduction in the thermalconductivity is caused; therefore, it is desirable for the thermallyconductive sheet to have good thermal conductivity, uniformity inthickness, and superior flexibility and shape follow-up property.

Such a thermally conductive sheet 10 is characterized by being composedof 10 to 25% by volume of carbon fibers and 40 to 55% by volume ofaluminum oxide (alumina). The thermally conductive sheet 10 is asheet-shaped product in which, for example, a silicone resin is used asits polymer, pitch-based carbon fibers are used as its thermallyconductive material and for example, spherical alumina is used as afiller, with these materials being blended with one another. Asdescribed earlier, the thermally conductive sheet 10 is formed byprocesses in which a thermally conductive composition having thepolymer, carbon fibers and alumina is allowed to pass through a slit sothat the carbon fibers are oriented in the extrusion direction, theresulting molded product is then cured to form a sheet base material 11,and the sheet base material 11 is sliced into a sheet form in adirection perpendicular to the extrusion direction of the sheet basematerial 11.

The silicone resin, which has superior physical properties, such asflexibility, shape follow-up property and heat resistance, is formed bymixing a first silicone resin and a second silicone resin with eachother. As the first silicone resin, polyalkenyl alkylsiloxane is used,and as the second silicone resin, polyalkyl hydrogen siloxane serving asa curing agent for the polyalkenyl alkylsiloxane is used.

Additionally, from the commercial viewpoint, the first silicone resin isavailable as a product in which a platinum catalyst serving as acatalyst in the above-mentioned reaction is mixed. Moreover, from thecommercial viewpoint, the second silicone resin is available as aproduct in which in addition to polyalkyl hydrogen siloxane, theabove-mentioned polyalkenyl alkylsiloxane and a reaction adjusting agentare mixed.

In the case when the first silicone resin and the second silicone resinare mixtures as described above, by simply blending the same amounts ofthese two resins with each other based upon weight ratios, it ispossible to make the compounding ratio of the first silicone resinrelatively higher, with the compounding ratio of the second siliconeresin serving as a curing agent being lowered.

As a result, it becomes possible to prevent the thermally conductivesheet 10 from being excessively cured, and consequently to generate aconstant compression rate. Since the thermally conductive sheet 10 isinterpolated between heat generating electronic parts and the heat sink,it needs to have a predetermined compression rate in the thicknessdirection so as to make these members in tightly contact with eachother; therefore, at least a compression rate of 3% or more, preferably6% or more, more preferably 10% or more, is desirably prepared.

Moreover, as shown in FIG. 13, the thermally conductive sheet 10 isdesigned such that the compounding ratio between the first siliconeresin and the second silicone resin is set to 55:45 to 50:50. With thisarrangement, even when thinly sliced to have the initial thickness of0.5 mm, the thermally conductive sheet 10 is allowed to have acompression rate of 3% or more (3.82%). Moreover, in the case of 52:48,the thermally conductive sheet 10 has a compression rate of 10.49% withthe initial thickness of 1.0 mm, and in the case of 55:45 to 52:48, ithas a compression rate of 13.21% with the initial thickness of 1.0 mm,both of which correspond to compression rates of 10% or more.

In this manner, since the thermally conductive sheet 10 has acompression rate of 3% or more in the thickness direction in spite ofits structure with carbon fibers oriented in the thickness direction, itis superior in flexibility and shape follow-up property, and allows theheat generating parts and the heat sink to be more tightly made incontact with each other, thereby making it possible to radiate heateffectively.

The pitch-based carbon fibers are composed of pitch as a main material,which is subjected to various treatments, such as a fusion-spinningprocess, an anti-fusing treatment and a carbonizing treatment, and thensubjected to a heating treatment at high temperatures in a range of 2000to 3000° C., or exceeding 3000° C., so as to be graphitized. Thematerial pitch is classified into isotropic pitch that is opticallydisordered with no bias being exerted and anisotropic pitch (mesophasepitch) in which constituent molecules are arranged in a liquid crystalstate, with an optical anisotropic property being exerted, and carbonfibers produced from the anisotropic pitch are more excellent inmechanical properties than those carbon fibers produced from theisotropic pitch, and have higher electrical and thermal conductivities;therefore, it is preferable to use these mesophase pitch-basedgraphitized carbon fibers.

Additionally, alumina has a particle size that is smaller than that ofcarbon fibers, and capable of sufficiently functioning as a thermallyconductive material, and is filled in tightly cooperation with carbonfibers. Thus, the thermally conductive sheet 10 can obtain sufficientpaths for thermal conductivity. As the alumina, DAW03 (made by DenkiKagaku Kogyo Kabushiki Kaisha) can be used.

<Compounding Ratio of Alumina and Carbon Fibers>

Depending on the compounding rate between the carbon fibers and alumina,the thermally conductive sheet 10 is varied in its evaluation in burningtests and evaluation as to easiness in extrusion at the time ofextruding a mixed composition composed of first and second siliconeresins, carbon fibers and alumina mixed with one another into arectangular pillar shape by using a syringe upon producing the sheetbase material 11 from which the thermally conductive sheet 10 is cutout. Additionally, the sheet base material 11 has its carbon fibersoriented in the longitudinal direction when it is allowed to passthrough a slit formed inside of the syringe, and after passing throughthe slit, is again molded into a rectangular pillar shape.

FIG. 14 shows the evaluation in burning tests (UL94V) of the thermallyconductive sheet 10 when the compounding rate of the carbon fibersrelative to 50 g of alumina is changed and the evaluation as to easinessin extrusion at the time of extruding the sheet base material 11 into arectangular pillar shape. Additionally, the thermally conductive sheet10 is formed by blending, as silicone resins, 5.4 g of the firstsilicone resin (mixture of polyalkenyl alkylsiloxane and platinumcatalyst) and 5.4 g of the second silicone resin (mixture of polyalkylhydrogen siloxane, polyalkenyl alkylsiloxane and a reaction adjustingagent).

As shown in FIG. 14, by blending 14 g or more of carbon fibers to 50 gof alumina, evaluations corresponding to V0 were obtained in the burningtests (UL94V) in both of the thermally conductive sheets 10 havingthicknesses 1 mm and 2 mm. Moreover, in accordance with the thermallyconductive sheet 10 having a thickness of 2 mm, by blending 8 g or moreof carbon fibers to 50 g of alumina, the evaluation corresponding to V0was obtained in the burning tests (UL94V). At this time, the volumeratio of 50 g of alumina in the thermally conductive sheet 10 was 45.8%by volume, and the volume ratio of 8 g of carbon fibers therein was13.3% by volume.

Moreover, by blending 8 g or 10 g of carbon fibers to 50 g of alumina,the thermally conductive sheet 10 is allowed to desirably maintain theeasiness of extrusion in the producing process of the sheet basematerial 11. In other words, the sheet base material 11 is allowed tosmoothly pass through the slit formed inside the syringe, and also tomaintain its rectangular pillar shape.

In the same manner, by also blending 12 g or 14 g of carbon fibers to 50g of alumina, the thermally conductive sheet 10 is allowed to desirablymaintain the easiness of extrusion in the producing process of the sheetbase material 11. In other words, the sheet base material 11 is allowedto smoothly pass through the slit formed inside the syringe, and also tomaintain its rectangular pillar shape. Additionally, the hardness ofthis sheet base material 11 is higher than that in the case of blending8 g or 10 g of the carbon fibers thereto.

In the case when 16 g of the carbon fibers were blended to 50 g ofalumina, the thermally conductive sheet 10 had its easiness in extrusionin the producing process of the sheet base material 11 slightlyimpaired. That is, since the sheet base material 11 was hard, there wasa problem in which one portion of the base material leaked through a jigfor use in securing the slit formed inside the syringe. However, it waspossible to allow the base material passed through the slit to maintainits rectangular pillar shape. At this time, the volume ratio of 50 g ofalumina in the thermally conductive sheet 10 was 40.4% by volume, andthe volume ratio of 16 g of carbon fibers therein was 23.5% by volume.

Moreover, in the case when 17 g of the carbon fibers were blendedthereto, the thermally conductive sheet 10 failed to be extruded in theproducing process of the sheet base material 11. That is, since thesheet base material 11 was hard, there was a problem in which oneportion of the base material leaked through a jig for use in securingthe slit formed inside the syringe. Moreover, base materials that hadpassed through the slit were not mutually combined with each other,failing to maintain its rectangular pillar shape.

Based upon the facts described above, it has been found that in the casewhen, in particular, a high flame retardant property corresponding to V0is required in the burning tests UL94V, the compounding amount of thecarbon fibers relative to 50 g of alumina is preferably set to 14 g inthe case of the sheet thickness of 1 mm, and also to 8 g to 16 g in thecase of the sheet thickness of 2 mm.

Moreover, as shown in FIG. 15, the compounding amount of the carbonfibers and the thermal resistance value are correlated with each other.As shown in FIG. 15, as the compounding amount of the carbon fibersincreases, the thermal resistance (K/W) is lowered; however, it is foundthat when the compounding amount becomes about 10 g or more, the thermalresistance value is stabilized. In contrast, in the case when 17 g ormore of the carbon fibers are blended thereto, since the extrusion ofthe sheet base material 11 becomes difficult as described earlier, thecompounding amount of the carbon fibers in the thermal conductive sheet10 is preferably set to 10 g or more to 16 g or less. In this case, withrespect to the thermally conductive sheet 10 having a thickness of 1 mm,the compounding amount of the carbon fibers was set to 14 g relative to50 g of alumina, from the viewpoints of flame retardant property of thethermally conductive sheet 10 and easiness in extrusion of the sheetbase material 11, and in this compounding amount, as shown in FIG. 15,the value of thermal resistance was kept low and stabilized.

Based upon the above-mentioned facts, as an example, FIG. 16 showscompounding ratios of a thermally conductive sheet 10 with a thicknessof 1 m that was produced based upon optimal compounding ratios (weightratios). As shown in FIG. 16, 5.4 g (7.219% by weight) of a mixture ofpolyalkenyl alkylsiloxane and a platinum catalyst was used as the firstsilicone resin, 5.4 g (7.219% by weight) of a mixture of polyalkylhydrogen siloxane, polyalkenyl alkylsiloxane and a reaction adjustingagent was used as the second silicone resin, 50 g (66.8449% by weight)of DAW03 (trade name) was used as alumina, and 14 g (18.7166% by weight)of R-A301 (trade name, made by Teijin Ltd.) was used as pitch-basedcarbon fibers.

<Slicing Device>

Next, the following description will discuss a structure of a slicingdevice 12 for use in slicing the sheet base material 11 into individualthermally conductive sheets 10 so as to obtain the thermally conductivesheet 10 having the compounding ratios shown in FIG. 16. As shown inFIG. 17, the slicing device 12 can produce the thermally conductivesheet 10, with the carbon fibers being oriented therein, by slicing thesheet base material 11 with an ultrasonic cutter 14. Therefore, theslicing device 12 makes it possible to obtain a thermally conductivesheet 10 having a superior thermal conductivity, with its carbon fibersbeing oriented in the thickness direction.

In this case, the sheet base material 11 is formed through processes inwhich, after the first and second silicone resins, alumina and carbonfibers have been charged into a mixer and mixed therein, the resultingmixture is extruded into a rectangular pillar shape having apredetermined size by the syringe formed in the mixer. At this time,when the sheet base material 11 is allowed to pass through the slitformed inside the syringe, the carbon fibers are oriented in thelongitudinal direction. After having been extruded into the rectangularpillar shape, the sheet base material 11 is put into an oven togetherwith the mold, and thermally cured and completed.

As shown in FIG. 18, the slicing device 12 is provided with a work table13 on which the rectangular pillar shaped sheet base material ismounted, and the ultrasonic cutter 14 that slices the sheet basematerial 11 on the work table 13, while applying ultrasonic wavevibrations thereto.

The work table 13 is provided with a silicone rubber 21 disposed on amovable carriage 20 made of metal. The movable carriage 20 is capable ofmoving in predetermined directions by a moving mechanism 22, andsuccessively transfers the sheet base material 11 toward the lowerportion of the ultrasonic cutter 14. The silicone rubber 21 has athickness that sufficiently receives the blade edge of the ultrasoniccutter 14. When the sheet base material 11 is mounted on the siliconerubber 21, the work table 13 allows the movable carriage 20 to move in apredetermined direction in accordance with the slicing operation of theultrasonic cutter 14, and successively transfers the sheet base material11 toward the lower portion of the ultrasonic cutter 14.

The ultrasonic cutter 14 has a knife 30 for use in slicing the sheetbase material 11, an ultrasonic oscillation mechanism 31 that appliesultrasonic wave vibrations to the knife 30 and a raising/loweringmechanism 32 that operates the knife 30 to be raised and lowered. Theknife 30 has its blade edge directed to the work table 13, and whenoperated to be raised and lowered by the raising/lowering mechanism 32,it successively slices the sheet base material 11 mounted on the worktable 13. The dimensions and material of the knife 30 are determineddepending on the size, composition, etc. of the sheet base material 11,and for example, it is made of steel with a width of 40 mm, a thicknessof 1.5 mm and a blade angle of 10°.

The ultrasonic oscillation mechanism 31, which applies ultrasonic wavevibrations to the knife 30 in a slicing direction of the sheet basematerial 11, has, for example, a transmission frequency of 20.5 kHz,with its amplitudes variably adjusted to 50 μm, 60 μm and 70 μm on threestages.

The slicing device 12 of this type successively slices the sheet basematerial 11 while applying ultrasonic wave vibrations to the ultrasoniccutter 14 so that the orientation of the carbon fibers of the thermallyconductive sheet 10 can be maintained in the thickness direction.

FIG. 19 shows thermal resistance values (K/W) of a thermally conductivesheet that was sliced without applying ultrasonic wave vibrationsthereto and a thermally conductive sheet 10 that was sliced whileultrasonic wave vibrations being applied thereto by the slicing device12. As shown in FIG. 19, it is found that, in comparison with thethermally conductive sheet sliced without applying ultrasonic wavevibrations thereto, the thermally conductive sheet 10 that was slicedwhile ultrasonic wave vibrations being applied thereto by the slicingdevice 12 has its thermal resistance (K/W) suppressed to a low level.

This is because of the fact that since the slicing device 12 appliesultrasonic wave vibrations to the ultrasonic cutter 14 in the slicingdirection, the interface thermal resistance is low so that the carbonfibers oriented in the thickness direction of the thermally conductivesheet 10 are hardly pushed down sideward by the knife 30. In contrast,in the case of the thermally conductive sheet sliced without applyingultrasonic wave vibrations thereto, since the orientation of the carbonfibers serving as a thermally conductive material is disturbed by africtional resistance of the knife, with the result that exposurethereof onto the cut surface is reduced to cause an increase in thethermal resistance. Therefore, by the use of the slicing device 12, itis possible to obtain a thermally conductive sheet 10 that is superiorin thermal conductivity.

<Slicing Rate and Uniformity by Slice Thickness>

Next, examinations were carried out on a relationship between theslicing rate of the sheet base material 11 by the slicing device 12 andthe thickness of the thermally conductive sheet 10 to be sliced. Byusing the compounding ratios shown in the aforementioned example (FIG.16), a rectangular pillar shaped base material 11 of 20 mm in each sidewas formed, and this sheet base material 11 was sliced into thermallyconductive sheets 10 having different thicknesses in a range from 0.05mm to 0.50 mm for each 0.05 mm, with the slicing rate of the ultrasoniccutter 14 being varied at 5 mm, 10 mm, 50 mm and 100 mm per second, andthe outside appearance of each of the thermally conductive sheets 10 wasobserved. In this case, the ultrasonic wave vibrations to be applied tothe ultrasonic cutter 14 had a transmission frequency of 20.5 kHz and anamplitude of 60 μm.

The results of the observation are shown in FIG. 20. As shown in FIG.20, under a thickness of 0.15 mm or less, a deformation was generatedirrespective of the slicing rate. In contrast, in the case of thethickness of 0.20 mm or more, no deformation in the thermally conductivesheet 10 was observed even when the slicing rate was increased. In otherwords, in accordance with the slicing device 12, it is possible touniformly slice the sheet base material 11 having the compounding ratiosshown in FIG. 16 with a thickness of 0.20 mm or more.

<Thermal Conductivity and Compression Rate in association with SlicingRate and Slicing Thickness>

Next, examinations were carried out on relationships among the slicingrate, the thermal conductivity and the compression rate in the thicknessdirection of the sheet base material 11 caused by the slicing device 12.With respect to the respective thermally conductive sheets 10 havingthicknesses of 0.20 mm, 0.25 mm, 0.30 mm and 0.50 mm formed at slicingrates at 5 mm, 10 mm, 50 mm and 100 mm per second, which had nodeformation in the examinations of the slicing rate and the sheetthickness, the thermal conductivity and compression rate thereof wererespectively measured. The results of measurements are shown in FIG. 21.

As shown in FIG. 21, among the respective thermally conductive sheets10, those thermally conductive sheets 10 except for a sample with asheet thickness of 0.50 mm had a superior thermal conductivecharacteristic in any of the cases in which it was sliced at any ofrates of 5 mm, 10 mm and 50 mm per second of the ultrasonic cutter 14,and also had a compression rate of 10% or more, thereby providingsuperior flexibility and shape follow-up property. Moreover, even in thecase when sliced at a rate of 100 mm per second of the ultrasonic cutter14, the thermally conductive sheets 10 having a sheet thickness of 0.25mm and 0.20 mm had a superior thermal conductive characteristic with acompression rate of 10% or more, thereby providing superior flexibilityand shape follow-up property.

In contrast, in the case of the thermally conductive sheet 10 having asheet thickness of 0.30 mm, when sliced at a rate of 100 mm per secondof the ultrasonic cutter 14, its compression rate was slightly loweredto 3.72%, although its thermal conductive characteristic was superior.

In the case of the thermally conductive sheet 10 having a sheetthickness of 0.50 mm, even when sliced at any one of rates of 5 mm, 10mm and 50 mm per second of the ultrasonic cutter 14, it is possible toprovide a superior thermal conductive characteristic, and a compressionrate of 5% or more, thereby making it possible to provide superiorflexibility and shape follow-up property. On the other hand, in the caseof the thermally conductive sheet 10 having a sheet thickness of 0.50mm, when sliced at a rate of 100 mm per second of the ultrasonic cutter14, its compression rate was lowered to 2.18% which was lower than 3%,although its thermal conductive characteristic was superior, causingdegradation in flexibility and shape follow-up property.

<Amplitude and Compression Rate>

FIG. 22 shows respective characteristics of thermally conductive sheets10 formed by being sliced, with the amplitude of ultrasonic wavevibrations to be applied to the ultrasonic cutter 14 being varied onthree stages of 50 μm, 60 μm and 70 μm. The thermally conductive sheets10 were formed based upon the compounding ratios shown in FIG. 16, withits measuring load being set to 1 kgf/cm². As shown in FIG. 22, in thecase when the amplitude was set to 70 μm, the resulting thermallyconductive sheet 10 had a compression rate of 2.18% that was lower than3% in the same manner as in the conventional structure, causingdegradation in flexibility and shape follow-up property. In contrast, inthe case when the amplitude was set to 50 μm and 60 μm, the resultingthermally conductive sheet 10 had a compression rate of 3% or more,making it possible to provide superior flexibility and shape follow-upproperty.

<Others>

Additionally, not limited to the rectangular pillar shape, the sheetbase material 11 may be formed into a pillar shape, such as a columnshape, having various cross-sectional shapes in accordance with theshape of the thermally conductive sheet 10. Moreover, although sphericalalumina was used as a filler agent, in addition to this, any one ofspherical aluminum nitride, zinc oxide, silicon powder and metal powdermay be used, or a mixture of these may also be used in the presentinvention.

INDUSTRIAL APPLICABILITY

In the case of a thermally conductive sheet produced by the method forproducing a thermally conductive sheet of the present invention, sincethe surface roughness on a cut surface is small to subsequently providea reduced thermal resistance, the thermal conductivity in the thicknessdirection becomes high; therefore, it can be desirably applied toperipheral parts of various devices, such as, for example, a CPU, anMPU, a power transistor, an LED, a laser diode, various batteries(various secondary batteries, such as lithium ion batteries, variousfuel batteries, and various solar batteries, such as wet-type solarbatteries, including capacitors, amorphous silicon, crystal silicon,compound semiconductors, etc.), which are adversely influenced inefficiency of element operations, service life, etc., depending on, forexample, temperatures; and the peripheral parts of heat sources ofheating apparatuses and the peripheral parts of heating pipes of heatexchangers and floor heating apparatuses in which heat needs to beeffectively utilized.

REFERENCE SIGNS LIST

1 . . . anisotropic thermally conductive filler, 2 . . . filler, 10 . .. thermally conductive sheet, 11 . . . sheet base material, 12 . . .slicing device, 13 . . . work table, 14 . . . ultrasonic cutter, 20 . .. movable carriage, 21 . . . silicone rubber, 22 . . . moving mechanism,30 . . . knife, 31 . . . ultrasonic oscillation mechanism, 32 . . .raising/lowering mechanism

1. A method for producing a thermally conductive sheet comprising atleast the steps of: by extruding a thermally conductive compositioncontaining a polymer, an anisotropic thermally conductive filler and afiller through an extruder, extrusion molding an extrusion moldedproduct in which the anisotropic thermally conductive filler is orientedalong the extrusion direction; curing the extrusion molded product toform a cured object; and cutting the cured object in a directionperpendicular to the extrusion direction into a given thickness with anultrasonic cutter.
 2. A method for producing a thermally conductivesheet comprising at least the steps of by extruding a thermallyconductive composition containing a polymer, an anisotropic thermallyconductive filler and a filler through an extruder, extrusion molding anextrusion molded product in which the anisotropic thermally conductivefiller is oriented along the extrusion direction; curing the extrusionmolded product to form a cured object; and upon cutting the cured objectinto a given thickness with an ultrasonic cutter, slicing the curedobject, with the cured object being disposed so that the anisotropicthermally conductive filler is oriented with an angle of 5° to 45°relative to the thickness direction of the cured object to be cut withthe ultrasonic cutter.
 3. The method for producing a thermallyconductive sheet according to claim 1 or 2, wherein the anisotropicthermally conductive filler has an average fiber length of 100 μm ormore.
 4. The method for producing a thermally conductive sheet accordingto claim 1, wherein the anisotropic thermally conductive filler isprepared as carbon fibers.
 5. The method for producing a thermallyconductive sheet according to claim 1, wherein the anisotropic thermallyconductive filler has a content of 16% by volume to 25% by volume in thethermally conductive composition.
 6. The method for producing athermally conductive sheet according to claim 1, wherein the filler hasan average particle size in a range from 1 μm to 40 μm.
 7. The methodfor producing a thermally conductive sheet according to a claim 1,wherein the filler is prepared as spherical alumina particles.
 8. Themethod for producing a thermally conductive sheet according to claim 1,wherein the polymer is a silicone resin.
 9. A thermally conductive sheetproduced by the method for producing a thermally conductive sheetaccording to claim
 1. 10. The thermally conductive sheet according toclaim 9, wherein a peripheral portion of the thermally conductive sheethas a slight stickiness that is higher than that of the inside of thethermally conductive sheet.
 11. The thermally conductive sheet accordingto claim 9, wherein the thermally conductive sheet has a cut surfacehaving a surface roughness Ra of 9.9 μm or less.
 12. The thermallyconductive sheet according to claim 9, wherein in the thermallyconductive sheet containing a silicone resin, a filler and carbonfibers, with the carbon fibers being oriented in the thicknessdirection, the filler is contained in a range from 40 to 55% by volume;and the carbon fibers are contained in a range from 10 to 25% by volume.13. The thermally conductive sheet according to claim 12, wherein thefiller is contained in a range from 40.4 to 45.8% by volume, and thecarbon fibers are contained in a range from 13.3 to 23.5% by volume. 14.The thermally conductive sheet according to claim 13, wherein 10 g ormore of the carbon fibers are contained relative to 50 g of the filler.15. The thermally conductive sheet according to claim 14, wherein 16 gor less of the carbon fibers are contained relative to 50 g of thefiller.
 16. The thermally conductive sheet according to claim 12,wherein the silicone resin contains polyalkenyl alkylsiloxane as a firstsilicone resin and polyalkyl hydrogen siloxane as a second siliconeresin, and the first silicone resin having a more amount than that ofthe second silicone resin is blended so that when cured by using aplatinum catalyst, a compression rate of 3% is exerted.
 17. Thethermally conductive sheet according to claim 16, wherein the filler isany one of aluminum oxide, aluminum nitride, zinc oxide, silicon powderand metal powder, or a mixture of two or more thereof.